Two questions arise.....
What time was Sun Set that day? It is reported to be 4:08 today thus it sounds as if darkness played a role.
What was the existing weather? The comment about "weather conditions" preventing SAR aircraft responding is also an indicator of how bad the weather was.
From that one can begin to consider what effect that had on visibility.
Put yourself in a 206 in those mountain valleys with night approaching and the weather not being very good, dead of Winter thus really cold, and you can begin to list the possible causes on a few fingers.
We shall have to wait for more definitive information before we can get past the mere speculation stage in this pprune accident investigation.